The problem of reduction in shipping generated SO2 is getting more serious and drawing people's attention world widely as related legislation is getting stricter. Shipping is an industry that correlates to the global economy, and in which a large amount of ships are equipped with oil-based fuel engines. Completely relying on using low-sulfur fuel oil may lead to dramatically rising cost. Hence providing shipping industry with economical flue gas desulphurization technology seems to be imperative.
Under mobile condition in different sea areas and with gas temperature as high as 200° C. to 490° C., compared to land fossil fuel fired industrial facility, the ship FGD technology must consider its practicality based on economic cost concern. Instead, it requires the total cost of ship FGD facility including manufacture cost and running cost to be significantly lower than the total saving cost of using low-sulfur fuel for substitute.
As a result, the IMO (International Maritime Organization) published the ship desulphurization regulation in 2005. Soon, the potential economic advantage of seawater FGD technology was recognized. In 2007, a research carried by scholars from four well known universities reached the conclusion: using the ocean resource, that is, the seawater, to realize ship SO2 reduction is a goal that people have pursued but have not realized.
The Alliance for Global Sustainability of four universities (Massachusetts Institute of Technology, University of Tokyo, Chalmers University of Technology, Swiss Federal Institute of Technology Zurich) published: “Seawater Scrubbing—reduction of SOx emissions from ship exhausts,” which points out: “It is shown that seawater scrubbing is a promising technology for reducing sulphur oxide emissions from ships . . . would therefore require detailed case studies . . . . Further studies would be needed in order to assess these options.”
Existing technologies that apply seawater scrubbing method to reduce ship exhaust are:
1. “A seawater scrubbing ship exhausts processing method” (Patent number 200710012371.1, published in China, Jan. 16, 2008). It uses a hollow fiber contactor as scrubber, and in this scrubber the ship exhaust undergoes dust removal and thereafter is scrubbed with seawater. The process is monitored by control system that includes an SO2 consistency sensor, water quality sensor, and PLC programming controller. The system may monitor and record the real-time SO2 consistency in processed ship exhaust, monitor and record discharged water quality, and control the water discharge, respectively. However, it has the problems described below:
1) The technology uses a hollow fiber contactor as scrubber. This hollow fiber contactor only tolerates decades of centigrade. It can not be used for high-temperature gas, especially for the ship engine exhaust with temperature as high as 200-490° C.
2) At the same time, pressure drop and resistance are very high when the hollow fiber contactor is used for scrubbing. Operation cost and energy expenditure would be quite high if additional booster fan is installed.
2. ‘Seawater Scrubbing—reduction of SOx emissions from ship exhausts’ (ISBN: 978-91-976534-1-1, American, 2007): “It is shown that seawater scrubbing is a promising technology for reducing sulphur oxide emissions from ships. Based on a 12 MW engine burning fuel with a 3% sulphur content. Calculations have been made for different efficiencies of sulphur scrubbing, different water temperatures, and for six different water types. The results of the calculations give the volumes of water required for (i) uptake of SOx (the scrubbing process), (ii) dilution of the scrubbing water to achieve a pH of 6.5, (iii) further dilution to achieve a pH within 0.2 units of that in the ambient water, and no more that a 1% reduction in dissolved oxygen concentration. The volumes of water required for a given efficiency of the scrubbing process . . . would require significantly larger water volumes for scrubbing and dilution . . . would therefore require detailed case studies. It may be possible to reduce the volumes of dilution water required by, for example, aeration of the scrubbing water and addition of base to neutralize the acidic sulphur oxides. Further studies would be needed in order to assess these options.”
The research is representative one in the seawater scrubbing method field for marine ship SO2 reduction. However, it remains in the stage of principle. It still fails to solve some problems: low absorb efficiency, large diluting water volume. To protect the environment from discharged water pollutant, EPA and IMO have passed regulations regarding the blending of discharged water with surrounding seawater. The blending process is defined as quick blending area and slow blending area. The boundary pH value is 6.5 for quick blending area, and 15 minutes is the time limitation for this value to be reached. For the slow blending area, the difference of boundary pH value and surrounding seawater pH value must less than 0.2. The report is carried based on a 12 MW engine. In order to fulfill the requirements regarding gas and discharge water, the ship must prepare thousands of tons of seawater per hour. Seawater needs further dilution after discharged from the ship and further blending with surrounding seawater that is not less than 40000 times of its volume.
This research has not mentioned the application of both technology and facility design.
3. Ecosilencer Exhaust Gas Cleaning Presentation (February 2006, Canada): Introduced EcoSilencer method seawater scrubbing system and equipment (FIG. 5). This technology experimented for more than 6 years until 2006. Core components correspond to the disclosure in U.S. Pat. No. 7,056,367: “Method and Apparatus for Scrubbing Gases Using Mixing Vanes” (FIG. 4).
The high-temperature gas from the engine must be cooled so that SO2 can be absorbed. For this reason, the scrubber of this invention (U.S. Pat. No. 7,056,367) adopts ‘exhaust gas inlet to liquid scrubbing tank,’ that is, the bubbling scrubbing method. However, the high-temperature gas cooling process and scrubbing process are carried out in the same tank at the same time. Hence, the transfer efficiency is quite low; in addition, the two processes conflict. In order to increase scrubbing efficiency under such condition, the only way is to increase the volume of scrubbing water and reduce pressure, which subsequently causes significant energy consumption and increase in operational cost. It seems to that the problems of a seawater scrubbing system that adopt this scrubber are caused by the conflict of cooling the high-temperature gas and absorbing SO2 at low-temperature, as well as the conflict of the scrubbing efficiency and reducing cost.
In conclusion, current research and experimental technology must confront similar issues when it comes into the practical field:
1) It is presently difficult to meet the environmental requirements of gas exhaust and seawater discharge under acceptable cost; energy consumption and operating costs are both very high to meet discharging water regulation because of the large volume of scrubbing water and diluting water; the cost will increase if higher requirements for discharging water are published in the future.
2) Unsuitable for mobility of ship. Different sea areas and controlling areas have different environmental conditions and governmental control requirements. Conditions change consistently for marine ship sailing between different sea areas and controlled areas. For example: seawater quality, sulphur content in different batches of fuel, discharging regulation in different environments, and other circumstances. It appears that flue gas desulphurization technologies with fixed parameters cannot be applied for ships.
Apparently, the expectation that using marine resource to eliminate air pollutant will not be realized if the mentioned problems remain unsolved.